1. Field of Invention
The invention relates to a buttonhole forming apparatus that is provided on a buttonhole sewing machine and forms a buttonhole on a work cloth by stitching a buttonhole and cutting a slit.
2. Description of Related Art
A conventional buttonhole sewing machine comprises a sewing device where buttonhole stitches are made on a work cloth, a cutter that makes a cut between buttonhole stitches made by the sewing device, a cutter driving mechanism that drives the cutter to move vertically, and a supporting device that supports the work cloth thereon having a cutter hole where the top of the cutter is inserted. A buttonhole forming apparatus is thought to be included in such a buttonhole sewing machine.
On this kind of buttonhole forming apparatus, a work cloth is held on the top of the supporting device, in which buttonhole stitches are made, the cutter vertically driven by the cutter driving mechanism is inserted through the work cloth into the cutter hole and then pulled out therefrom. Thus, a cut is made between the buttonhole stitches made by the sewing device, and finally a buttonhole is formed.
So far, a cutter driving mechanism has been arranged in most cases that a driving force of the machine motor is mechanically transmitted to the cutter via a link system, and the cutter cuts the work cloth.
However, application of such a mechanical cutter driving mechanism to a machine provides the cutter with the necessary driving force, but the operation of the cutter is slow, therefore the reduction of the cycle time of the machine is longer than desired.
In addition, currently being developed by the assignee of the instant application, is an air cylinder that can be used to drive the cutter driving mechanism. In this case, the operational speed of the cutter is increased, but the cutting performance of the cutting device cannot be fully assured. Moreover, in another ongoing development, the cutter driving mechanism is driven by a solenoid, the operational speed of the cutter would be expected to be faster, but the cutter may not return to its original position, as it may be kept inserted into the work cloth.
For example, as shown in FIG. 11, the needle plate 151 having a cutter hole 167 with the same width as the thickness of the cutter is disposed on the bed of the buttonhole sewing machine as the supporting device. A buttonhole is formed on a work cloth placed on the top 151a of the needle plate 151. In this case, the sewing device, such as a needle (not shown), forms buttonhole stitches in ranges indicated by A on both sides, a top of the cutter 13 is inserted into the cutter hole 167 to cut the work cloth, and a buttonhole is defined. However, as is obvious from FIG. 11, the work cloth W may be caught between the cutter hole 167 and the edge of the cutter 13, and the cutter 13 may not be returned to its original position only by the action of an air cylinder or a restoring spring of the solenoid (not shown).
This problem of catching of the work cloth is thought of as the use of an air cylinder or solenoid cannot secure the required cutting performance. Therefore, if the cutter driving mechanism is arranged using the air cylinder or the solenoid, the cycle time can be reduced but buttonholes can not be correctly formed on a thicker material.